MEANS FOR REDUCING SELF-DISCHARGE IN A ZINC HALIDE STORAGE CELL

Abstract: A rechargeable zinc halide electrolyte electrochemical cell including a laminate of the sequence a) an electrically conducting chemically inert material, b) a bilayer membrane matrix formed from a pair of hydrophilic polymer matrices in each of which is immobilized zinc halide electrolyte, c) an electrically conducting layer adapted to adsorb and absorb halogen, and d) a layer capable of reversibly absorbing and releasing bromine disposed between the membrane matrices for reducing self-discharging in the cell.

Description: MEANS FOR REDUCING SELF-DISCHARGE IN A ZINC HALIDE STORAGE CELL TECHNICAL FIELD The present invention relates to rechargeable zinc halide cells and methods of making the same. Such cells can be used individually or connected electrically in series to form a non-flow zinc halide storage battery. BACKGROUND OF THE INVENTION A thin ribbon like flexible rechargeable zinc halide cell is disclosed in U. C Patent 5,011,749 to applicant, issued April 30,199-L, the disclosure of which being incorporated herein by reference. The'749 patent specifically discloses a rechargeable zinc halide electrolyte electrochemical cell comprising at least a three layer laminate. A first layer includes an electrically conducting chemically inert material. A second layer includes a membrane matrix formed of a hydrophilic polymer matrix in which is immobilized zinc halide electrolyte. A third layer includes an electrically conducting layer adapted to absorb and adsorb halogen. The disclosed cells, having various embodiments in addition thereto, provided a quite satisfactory cell having reduced hydrogen evolution and dendrite formation while providing sufficient energy density without requiring an overdesign in zinc capacity. As discussed above zinc halide and especially zinc bromide storage cells have been developed as flow cells. One of the reasons for this is the fact that in zinc bromide cells, the dissolved bromine reacts chemically with the zinc formed at the opposite electrode causing self-discharge. In a flow cell where the electrolyte solutions are circulated, there is the possibility of storing bromine in a separate container. On standing, there is no chemical contact between the bromine and the zinc. Flow systems are more expensive, however, than non-flow systems and a bromine containing liquid is circulating all the time in flow systems. This is a great security risk, especially in electric vehicles. Although it has been previously attempted to produce non-flow batteries these trials failed mostly because of the self-discharge problem. It has been attempted to insert expensive cation membranes to retain the bromine somewhat, but self-discharge still was of the order of 50% in two days. The'749 patent discloses a system where the bromine is absorbed on a conducting layer adapted to absorb and adsorb halogen. This combined with a matrix which supports the electrolyte. In such a system the selfdischarge is greatly reduced, but still appreciable. The present invention provides a discovery which greatly reduces self-discharge while maintaining effective charge and current carrying capabilities. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a rechargeable zinc halide electrolyte electrochemical cell including a laminate of the sequence: a. an electrically conducting chemically inert material, b. a bilayer membrane matrix formed of a pair of hydrophilic polymer matrices in each of which is immobilized zinc halide electrolyte, c. an electrically conducting layer adapted to adsorb and absorb halogen, and d. a layer of a material capable of absorbing and releasing bromine disposed between the membrane matrices for reducing self-discharge in the cell. DETAILED DESCRIPTION OF THE INVENTION A rechargeable zinc halide electrolyte electrochemical cell constructed in accordance with the present invention generally includes a piece of electrically conducting cloth, for instance made of carbon or graphite fibers, which is covered with a mixture of activated carbon and electrolyte solution. Carbon blacks of high surface area, such Ketjen black or Black Pearls are preferred. A preferred composition of electrolyte solution is 3.5M zinc halide and 3M potassium chloride. On this composition, a sheet of hydrophilic polymer in either a crosslink or non-crosslinked form is deposited. A suitable polymer which is preferred for the present invention is polyacrylamide. A battery separator is then laid, preferably carrying acidic groups such as Permion 1010. On this again, a piece of polymer sheet is deposited and on that another piece of conducting graphite cloth. This sandwich is thoroughly wetted with electrolyte solution. The thus obtained rechargeable cell can be pressed between additional current collectors or may be enclosed in a plastic envelope with pieces of the graphite cloth protruding for making electrical contact. Such a cell is approximately two millimeters in thickness and can be charged for two hours at approximately 8mA/cm2 and will discharge for about two hours at about 80% current efficiency and about 95% voltage efficiency. Cycling can be pursued for hundreds of times without any change in the current voltage behavior as a function of time. Energy density conforms to 120mWH per mol. t As discussed below a wide choice of polymers and separators can be used and the choice is made according to considerations of price and performance. The greater part of the halogen formed is retained in the activated carbon layer. The activated carbon can be mixed with anion exchange material. The remainder of the cell is only in contact with a very small amount of halogen and corrosion problems are much smaller than those encountered in flow cells. Preferably, the electrolyte includes a bromine complexing agent. The addition of the bromine complexing agent unexpectedly provides the beneficial effect of decreasing corrosion after prolonged cycling and decreasing cell selfdischarge. More specifically, one of the advantages of the non-flow cell as compared to the flow cell is the absence of corrosion because there is no flowing bromine containing stream. But even with the non-flowing system, after prolonged cycling there is some corrosion that can be found at the bromine electrode. The addition of complexing agent unexpectedly prevents this corrosion entirely. That is, the bromine complexing agent unexpectedly functions in a manner which is totally different than that expected from its use in prior art flow cells. Hence, the otherwise unnecessary additions of the coupling agents unexpectedly decreases electrode corrosion and slightly provides additional protection against unwanted self-discharge. With specific regard to the bromine complexing agent, the bromine complexing agent can be included in the range 0.001 to 4 moles/liter. The bromine complexing agent is selected from the group consisting essentially of N-ethyl, N-methylmorpholinebromide, N-ethyl, N-methylpyrrolidiumbromide, N-methoxymethyl, N-methylpiperidiniumbromide, N-chloromethyl, N-methylpyrrolidiniumbromide, all belonging to the group of unsymmetrically substituted quaternary ammonium salts. Other complexing agents capable of functioning in accordance with the present invention can be used. As stated above, a further aspect of the present invention relates to the preparation of the polymer matrix in the non-flow zinc halide cell. Theproblem addressed is a fact that an aqueous solution of the acrylamide in water and in the presence of crosslinker readily polymerizes with a persulfate initiator but polymeration will not occur if electrolytes are also dissolved in the mixture. This chemistry prevents the in situ preparation of the polymer matrix and requires preparation of polymer matrix and then the addition of the electrolyte after polymerization and drying has occurred. Swelling results from the addition of electrolytes which causes a loss of precision in sizing of the matrix. Applicants have found that it is possible to perform the polymerization of the matrix in the presence of redox initiator. Generally, this aspect of the present invention provides a method of making the polymer matrix of the rechargeable zinc halide electrolyte electrochemical cell by polymerizing a solution comprising an acrylic monomer in the presence of a redox initiator and a zinc halide electrolyte to form the polymer matrix. More specifically, it is well known that acrylic monomer can be polymerized by redox initiators. In accordance with the present invention, however, the polymerization occurs readily in the presence of electrolytes but fails when only monomer, initiator and crosslinker are present in the absence of the electrolyte. Accordingly, applicant has discovered a novel and inexpensive synergy between the monomer, redox initiator, and electrolyte. The present method lends itself to in situ preparation of the polymer matrix in a cell. That is, the acrylic monomer, redox initiator, and zinc halide electrolyte solution can be poured into a cell and polymerization of the solution take place in situ to totally conform with the cell dimensions. This was not achieved by prior art methods of addition of the electrolyte to a polymer matrix resulting in swelling of the matrix. Acidification of the solution of the acrylic monomer, redox inhibitor, and zinc halide is beneficial. Acidification is accomplished by bringing the pH of the solution a range of pH 1 to 5. Preferably, the solution is brought to a pH of about 2. Preferably acids for acidifying the solution are selected from the group consisting essentially of hydrochloric acid, sulfuric acid, phosphoric acid, trichloroacetic acid and trifluoro acetic acid. Acrylic monomers which are preferred for the present invention are selected from the group consisting essentially of acrylamide, acrylic acid, methacrylamide, vinylpyrrolidone, and polyethyleneglycolacrylates. The oxidative part of the redox initiators which are preferred for use with the present invention are selected from the group consisting essentially ofpersulfates of sodium, potassium, or aluminum; t-butyl peroxide, hydrogen peroxide; benzoyl peroxide; and methylethylketone peroxide and the reductive parts from bisulfate, metabisulfate (pyrosulfite); dithionate; ascorbic acid; sodium thiosulfate; and sodium formaldehyde sulfoxylate. Preferably, the solution which is used to form the polymer matrix of the present invention comprises a. 1 to 10 molar acrylamide, b. 0.0001 to 1 molar redox initiator, and c. 0.1 to 6 molar zinc halide, and d. 0.1 to 6 molar complexing agent. The solution for preparing the polymer preferably includes an additional halide salt such as potassium chloride and ammonium chloride preferable 0.1 to 6 molar potassium chloride is used. Polymerization preferably occurs at a temperature of 0 to 90 C and can be accomplished in about 30 minutes or less. As discussed above, the polymer matrix can be laminated between the electrically conducting chemically inert material and an electrically conducting layer adapted to adsorb and absorb halogen to form the electrochemical cell. Such cells can be connected in series to form a rechargeable zinc halide electrolyte chemical battery. In the preferred embodiment of the present invention, it has been found that self-discharge can be greatly reduced by the use of two instead of one polymer matrix. The cell includes a laminate having a bi-layer membrane matrix formed of a pair of hydrophilic polymer matrices in each of which is immobilized the zinc halide electrolyte. Critically, a material capable of reversibly adsorbing and releasing bromine is disposed between the membrane matrices for reducing self discharge in the cell, such as an anion exchanging material. The anion exchange layer preferably includes at least one layer of anion exchange beads separating the membrane matrices. It is critical that the layer clearly separate between the membrane matrices. It has been found that if the anion exchange beads are not forming a separate layer but are homogeneously distributed inside the matrices, almost no effect is observed. On the other hand, by the use of the layer of anion exchange beads or other materials in accordance with the present invention, selfdischarge has been reduced to a decrease of 15% of total discharge within 48 hours, which is a value accepted by the guidelines of the U. S. battery consortium. Preferably, the anion exchange beads are macroreticular resins. Stongly basic types such as Amberlite IRA 900 with quaternary ammonium functionality work well, but also the weaker types with polyamine functionality like Amberlite IRA 93 appeared to be quite satisfactory. These are Rohm and Haas products. Equivalent resins can be obtained from other manufacturers like the Dowex resins from Dow chemical, which work as well. Preferred bead sizes are from 14 to 400 mesh. As other materials can be mentioned phosphinic and Arsinic compounds. Triphenylphosphine for instance could be shown to prevent selfdischarge as well as the IRA 900 resin. An especially active and resistant example of such exchangers is Amberlite IRA 93 resin and similar resins made by other companies. A bi-polar non-flow zinc halide storage battery can be made by a plurality of cells constructed as above being electrically connected in series, each of the cells being a rechargeable zinc halide electrolyte electrochemical cell as described above. In batteries such as discussed above, it is not advisable to use metal current carriers because of corrosion problems and it is better to use a carbon/plastic composite material. In non-flow zinc halogen batteries such as those constructed in accordance with the present invention, many cells are coupled in bipolar fashion and it is important that the separating charge carrier be as thin and conductive as possible. The preferred embodiment of the present invention includes a novel current carrier comprising a first layer which is electrically conductive and a pair of plastic foil sheets which sandwich the first layer therebetween. Such layers can be made to be a few hundred microns thick thereby making them thin and light and therefore preferable for use as current carriers for storage batteries conducted in accordance with the present invention. Since there are no great demands on the dimensional stability of the carrier, such extremely thin and light current carriers are more than sufficiently functional in systems constructed in accordance with the present invention. The first layer which is sandwiched between the plastic foil sheets can be a material selected from the group consisting essentially of carbon fiber paper, carbon felt, carbon cloth, carbon paper, and carbon fibers, wherein carbon is used to mean carbon in all of its forms, such as graphitic, amorphous, microcrystalline, diamond or fullerene. The thickness of the material can be 10 to 10000 microns. Preferably, carbon paper is used which is only a few hundred microns thick and is electrically conductive. The plastic foil sheets comprise a material selected from the group consisting essentially of polyethylene, high as well as low density, polypropylene, r polyvinylacetate, polyvinylchloride, chlorinated polyethylene, and mixtures or copolymers thereof. A preferred method of making the current carrier described above includes the steps of disposing the first layer of material which is electrically conductive and between a pair of plastic foil sheets and pressing the foil sheets together to sandwich a first layer therebetween. This process is accomplished at a preferred temperature for a defined period of time. The pressing step can be accomplished at a temperature of 25 to 400 C for one to 600 minutes at a pressure of 14 to 50000 psi. Preferably, the temperature and time for pressing will be adjusted with respect to the thickness of the plastic foil sheets and first layer of material as well to the composition of the materials. Such adjustments can be accomplished by those skilled in the art having an understanding of the present invention as described above. The pressing of the materials can be done in a static arrangement using a single press or can be continuously done by passing the aligned layers between heated rolling drums of a pinch press. In use in a bipolar battery there are two aspects of the current carrier of critical importance. The current carrier must conduct current from cell to cell through the carrier, this property being expressed as a resistance in ohms per square centimeter. This can be referred to perpendicular conductivity. This perpendicular conductivity must be less than 1 ohm/cm2 for there not to be caused unnecessary losses. A second critical feature is the lateral conductivity, which is important in order to make a charging or discharging correction on one cell in a battery stack. This lateral conductivity is expressed as a resistance in ohms/square. There is no unit for the square because every square for a certain material has the same lateral resistance when measured along the entire side of the square. Less than 10 ohms per square is a reasonable value for this resistance. With regard to the specific current carrier of the present invention, a conducting material is pressed between foil layers. If a metal sheet is pressed between foils, there would never be perpendicular conductivity because the foils insulate the metal effectively. With regard to the present invention, it is presumed that the hairy or rough surface of the carbon paper or felt penetrates the plastic foils during the hot pressing step and are exposed enough at the surface to provide the electrical contact. With a prior art system of hot pressing, a mixture of carbon powder and plastic, conductivity is caused because the carbon particles touch each other and apparently also penetrate through a bit from the plastic surface after hot pressing so that they can take up the current. In the present invention, the hairs or projections from the carbon or felt paper presumably function in a similar manner. The exact opposite occurs with lateral conductivity. If for perpendicular conductivity the resistance decreases with thinner materials, the perpendicular conductivity prefers a thicker material to provide a lower resistivity. In the classical material with the carbon powder, lateral conductivity mostly is not sufficient and some metal screen is incorporated which will take care of the lateral conductivity. Unexpectedly provided by the present invention is in the lateral direction there is a continuous layer of carbon paper or felt and this layer is conductive enough to provide low resistance also in the lateral direction. In view of the above, only conducting foils which have a certain roughness, so that after pressing parts of the conducting material such as fibers or the like are still penetrating out to the plastic layers, will show perpendicular conductivity. On the other hand, the fact that there is a continuous conducting foil in the lateral direction provides good lateral conductivity. The following examples illustrate a usefulness and unexpected results of the present invention. EXAMPLES: 1. A solution was prepared in ion free water of 3.5M zinc bromide, 3 M potassiumchloride and 1 M N-ethyl, N-methylmorpholine bromide. This solution after having been brought to pH 2 with concentrated hydrochloric acid was the electrolyte solution. Cabot's Black Pearls activated carbon (300 mg) was thoroughly wetted with the solution and pressed to a thickness of 1 mm. A 12% solution of monomer in the electrolyte solution with 1% crosslinking agent is polymerized on top of the carbon layer to a thickness of 1mm in combination with a redox initiator. After completion of the polymerization, a layer of 300 mg of Amberlite IRA 93 was spread on the polymerized layer and another 1 mm thick layer of polymer is put on top. Schwanigan activated carbon (300 mg) which was wetted thoroughly with the electrolyte solution was placed on top of that also 1 mm thick. After the cell is closed, it was cycled for a few times and it was charged until it contained 400 mAh. After 48 hours standing at open circuit, it was discharged and gave 320 mAh, 80 mAh having been lost by selfdischarge. 2. The same experiment was repeated but now with all layers being 1.5 mm thick instead of 1 mm. This cell was charged up to 600 mAh and retained after two days 510 mAh. 3. The cells of examples 1 and 2 were prepared without the ion exchange beads and both lost about 50% of their charge in 48 hours. 4. A solution was prepared in ion free water of 3.5M zinc bromide, 3 M potassiumchloride and 1 M N-ethyl, N-methylmorpholine bromide. This solution after having been brought to pH 2 with concentrated hydrochloric acid was the electrolyte solution. Cabot's Black Pearls activated carbon (300 mg) was thoroughly wetted with the solution and pressed to a thickness of 1 mm. A 12% solution of monomer in the electrolyte solution with 1% crosslinking agent is polymerized on top of the carbon layer to a thickness of lmm in combination with a redox initiator. After completion of the polymerization, a layer of 300 mg of finely powdered triphenylphosphine crystals were spread on the polymerized layer and another 1 mm thick layer of polymer is put on top. Schwanigan activated carbon (300 mg) which was wetted thoroughly with the electrolyte solution was placed on top of that also 1 mm thick. After the cell is closed, it was cycled for a few times and it was charged until it contained 400 mAh. After 48 hours standing at open circuit, it was discharged and gave 320 mAh, 80 mAh having been lost by selfdischarge. The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims: What is claimed is: 1. A rechargeable zinc halide electrolyte electrochemical cell comprising a laminate of the sequence: a. an electrically conducting chemically inert material; b. a bilayer membrane matrix formed of a pair of hydrophilic polymer matrices in each of which is immobilized a zinc electrolyte; c. an electrically conducting layer adapted to adsorb and absorb halogen; and d. a material capable of reversibly absorbing and releasing bromine in the form of at least one layer disposed between said membrane matrices for reducing self-discharge in said cell. 2. A cell as set forth in claim 1 wherein said aborbing and releasing bromine means includes at least one layer of anion exchange beads separating said membrane matrices. 3. A cell as set forth in claim 2 wherein said anion exchange beads are macroreticular resins. 4. A cell as set forth in claims 1-3 including addition membrane matrices between layers (a) and (c) and at least one layer of said anion exchange means disposed between each of said membrane matrices. 5. A cell as set forth in claim 1 further including a sublayer (b) adjacent said layer (c) which comprises a porous membrane matrix. 6. A cell as set forth in claims 1-5 which can be cut to any desired size of battery, wires being attached to the final size assembly to layers (a) and (c). 7. A bipolar non-flow zinc halide storage battery comprising a plurality of cells electrically connected in series, each of said cells being a rechargeable zinc halide electrolyte electrochemical cell as set forth in claims 1-6.

Inventors: MANASSEN JOOST, MANASSEN, JOOST
Applicants: MANASSEN JOOST, RYCUS AVIGAIL, YEDA RES & DEV, MANASSEN, JOOST, RYCUS, AVIGAIL, YEDA RESEARCH AND DEVELOPMENT CO., LTD. AT THE WEI
Application year: 1994
Main classification: H01M12
IPC codes: H01M2, H01M6, H01M12